1. Technical Field
The present invention relates to a defect detecting method for automatically detecting a low-contrast defect with accuracy in inspection processes for various products such as an inspection process in manufacturing display devices and projectors, the display devices including a liquid crystal display panel and the projectors being applied products of liquid crystal display panels. The invention also relates to a defect detecting device.
2. Related Art
It has been difficult for a detecting device to automatically detect defects such as a blur defect, an unevenness defect, a luminescent spot and a black spot in an inspection for a liquid crystal display panel such as a TFT (Thin Film Transistor) panel, since such defects generally have different shapes and low contrast. Hence, the inspection is still now conducted by a checker visually, which evokes pressing demands for automating the inspection to reduce the manufacturing cost.
Note that when a certain region of a display screen has a different luminance from the other, in other words, when a certain range is brighter or darker as compared with the surrounding portion, such states are called as a blur defect or an unevenness defect. Generally, when the planar dimension of such defect is relatively small, the defect is called a blur defect. When the planar dimension is relatively large, the defect is called an unevenness defect. When the planar dimension is small to be a spot, the defect is called a luminescent spot or a black spot.
As a method for automating a defect detection for an inspection object such as a liquid crystal display panel that has pixel portions and black matrix portions defining pixels, the pixels being transmitting light to be turned on and the pixel and black matrix portions disposed in an identical sequence of pattern, an eight-adjacent-point comparing method for detecting a defect using pixel data at eight points around the inspection object has been known (see, for example, Document: JP-A-2004-28836).
The document discloses that whether there is a defect or not is judged by selecting two points that are optimum to be used in a comparison with the inspection object out of the eight adjacent points (disposed on the left, right, top, bottom, obliquely upper and lower left and obliquely upper and lower right of the inspection object) and making the comparison between an average value of the selected two points and luminance data of the inspection object. When each luminance data of the two points are the same, such pair of points is judged to be optimum points for the comparison with the inspection object.
However, according to the Document, since one of the two points is in a black matrix portion and the other is in a pixel portion at, for example, one of the four corners of a liquid crystal display panel, the two points disposed with the inspection object interposed, no pair of points can have common luminance data. Accordingly, it is not possible to select two points optimum for the comparison, so that a simulated defect may be detected or the corners may be mistakenly judged to be out of an inspected range, thereby hindering an accurate defect detection at the corners.
As another problem, since the inspection object and the two points to be compared with the inspection object all need to be in either a pixel portion or a black matrix portion, a pitch of the inspection object and the comparison points need to be in line with that of the pixels, thereby complicating the inspection.